This invention relates to oil shale and more particularly to continuously removing arsenic from shale oil with a catalyst and regenerating the catalyst in situ without interrupting operations.
Researchers have now renewed their efforts to find alternative sources of energy and hydrocarbons in view of past rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing materials such as oil shale, coal and tar sands by pyrolysis or upon gasification or by solvent extraction, hydrolysis, beneficiation and retorting or combinations thereof, to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah and Wyoming.
The remainder is contained in the leaner Devonian-Mississipian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to improved methods of processing shale oil. One problem encountered in such processing is the necessity for removing contaminants or impurities which would poison expensive catalysts, such as platinum catalysts and the like, that are used to upgrade light fractions of shale oil and other hydrocarbonaceous fluids before they can satisfactorily be used as a source of energy.
Arsenic is one of the components of raw shale oil which is present at several orders of magnitude higher concentration than in conventional crude oil, and which acts as a reforming and hydrotreating catalyst poison because of its affinity for the metals platinum, cobalt, nickel, and the like. Arsenic also absorbs on the active sites of the hydroprocessing catalysts used to remove nitrogen from whole shale oil. Since arsenic poisons most of the catalysts currently used for refining oil, the arsenic in shale oil must be removed before shale oil can be piped into a conventional refinery. Generally, it is desirable to reduce the level of arsenic in the naphtha fraction to 5 ppb or less before catalytic reforming. Even if the shale oil is employed directly as a fuel, removal of such contaminants may be desirable from the environmental standpoint. Thus, it is desirable that contaminants such as arsenic be removed or reduced to low levels prior to use or further processing.
Generally, oil shale is a fine-grained sedimentary rock stratified in layers with a variable richness of kerogen content. Kerogen has a limited solubility in ordinary solvents and therefore may not be economically recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, propane, and propene, as well as other products such as hydrogen, carbon dioxide, carbon monoxide, ammonia, steam and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil", is the liquid oil product recovered from the liberated effluent of an oil shale retort.
Untreated shale oil contains various contaminants such as nitrogen, sulfur, oxygen and trace metals, such as arsenic, iron, vanadium and nickel. Aresenic is generally present at levels of from 2 to 80 wppm in full range shale oil, and at levels of from 20 to 1200 wppm in many coal tar distillates. These contaminants must be substantially removed from the shale oil in order to produce a marketable, high quality oil product. Further, several of the trace elements found in whole shale oil, including arsenic, can adversely affect operation with both conventional cracking and hydrogenation catalysts. Most of the trace elements in shale oil are concentrated in the heavy end. However, arsenic is distributed throughout the boiling range of the raw shale oil and various fractions imply the presence of organic arsenic compounds, although the nature of arsenic contained in shale oil is not fully understood. One study suggests that about half of the total arsenic in shale oil is in organometallic form and the other half is in inorganic form. Some of the organometallic arsenic compounds are thermally unstable and volatilize upon heating and become part of the lighter liquids. Therefore, the majority of the arsenic must be removed by other means.
Many different arsenic removal processes have been studied and employed. The most promising and widely studied and used is the fixed bed catalyst process in which the retorted shale oil is passed through a fixed guard bed which contains a catalyst capable of removing or reducing the arsenic content of shale oil to acceptable levels. The guard bed is generally a pressure vessel adapted to withstand the temperatures and pressures needed to remove elemental or combined arsenic and is charged with a suitable catalyst capable of removing the arsenic from the shale oil. See U.S. Pat. Nos. 3,876,533; 4,003,829; 4,046,674; 4,188,280; 4,051,022; 3,933,624; 3,954,603; 3,093,574; 4,446,006; 4,069,140; 4,424,118; 4,141,820; and 4,075,085.
Regardless of the catalyst or process conditions employed, plugging problems have been encountered when the average arsenic deposited on the catalyst is between five and ten percent, generally seven percent, of the catalyst weight resulting in a relatively short catalyst life and necessitating shutting down the dearsenation process while spent catalyst is removed and the guard bed charged with fresh catalyst.
A further drawback to prior art methods is the problem of disposal of the spent catalyst. After a period of time, for example two months, depending upon the catalyst and the arsenic content of the feed, the catalyst becomes saturated with arsenic and does not continue to absorb that contaminant. At this stage, breakthrough occurs and the arsenic contained in the feed reaches the main hydroprocessing reactors and poisons the catalysts employed therein.
Removal of the spent catalyst creates several problems. The first is the downtime involved with the removal of spent catalyst, loading the bed with fresh catalyst and preparing the fresh catalyst bed for operation. The second and equally serious problem is that of waste disposal. Arsenic-containing catalysts are considered to be hazardous waste products, and pose a serious disposal problem. It is generally desirable to avoid combusting spent catalyst for environmental reasons in order to avoid polluting the atmosphere. In order to be disposed of as non-hazardous waste, the arsenic content of the spent catalyst must be reduced to about 0.3 weight percent prior to disposal, and the aqueous solubility of arsenic, as measured by the standard EPA Toxicity test must be less than 5 ppm.
Careful investigation has shown that thus far, attempts to passivate or fix the arsenic on the catalyst in an insoluble form to successfully pass the EPA Toxicity test have not succeeded. The only other alternative is to regenerate the catalyst so that it may be reused, rather than being disposed of.
The prior art suggests several methods for regenerating catalysts. U.S. Pat. No. 4,272,400 discloses a method of regenerating spent alumina catalyst supports impregnated with Group VIB and VIII metals used to reduce sulfur content in an acid media and sulfurous atmosphere and at temperatures of from 400.degree.-825.degree. C. Acid media are unsuitable, however, to regenerate spent arsenic guard bed catalysts, as the acid attacks the catalyst itself.
U.S. Pat. No. 3,761,400 discloses a method of catalyst rejuvenation which entails tumbling the catalyst pellets or particles to grind away the outer surface. Not only does this process require removal of the catalyst from the guard bed, and hence interruption of the operation, but its use is limited and the method cannot be employed repeatedly without destroying the catalyst.
U.S. Pat. No. 4,227,027 discloses a method of reactivating arsenic-poisoned noble metal catalysts by purging the catalyst with an arsenic-free gas such as arsenic-free ethylene combined with acetylene. This method is not suitable for the present purposes.
The present invention fulfills a long standing need by providing a method of continuously removing arsenic from shale oil and regenerating the spent catalyst in situ without interrupting operations, or requiring fresh catalyst. Thus, the process of the present invention eliminates down-time, extends the life of the guard bed catalyst, reduces costs, and satisfactorily solves the problem of arsenic removal and disposal.